This invention relates to the field of calendering machines and in particular to super calenders of the type used to finish paper webs for printing or other applications where a relatively high smoothness is required. In super calenders the paper web passes between the nips of a number of rollers pairs which, by application of circumferential friction, effect a polishing action on the web.
The rolls are generally arranged in a vertical stack with iron rolls alternating with paper filled or similarly compliant rolls.
The function of the super calender is two-fold. First, it is desired to uniformly size the web, that is, to set its caliper. Second, it is desired to polish the web while maintaining caliper and bulk.
Super calenders are fairly large machines having up to twenty rolls. The applied pressure plus the weight of the stack of rolls, therefore, is quite significant, causing deformation in the geometric profile of the rolls in the stack. Such deformities cause undesirable variations in the web in both bulk and caliper, as well as affecting polishing action.
To deal with this deformation rolls are often crowned so that they are slightly larger in the center than the ends or quarterpoints, as they are known in the art. This technique alone is insufficient to deal with rolling variation and uneven wear as the web passes through the super calender with the result that hot spots occur and web quality and dimension are adversely affected. A hot spot will occur at varying points along the nip of each pair of rolls. A hot spot results from excessive nip pressure causing a temperature increase. This excessive heat causes thermal crowning, a condition that breeds on itself.
In an effort to improve web handling, variable crown rolls have been developed and are often referred to as zone control rolls. Such rolls permit selective deformation of the surface of the roll (the shell) to control its geometric profile across the area of the nip. See, for example, U.S. Pat. No. 4,327,468.
Zone control rolls usually employ internal hydraulic elements which are actuable, in groups or individually, to produce a variable geometry crown. As detailed in the prior art statement, it is known to employ a zone control roll for the top nip of a calender stack and to control its geometric profile as a function of web caliper. In particular, this was accomplished by applicants' assignee in an actual installation in the United States in 1979. This installation, while providing improved (more uniform) web caliper, did not entirely solve the problem.
In general, the caliper of a web is determined by the pressure applied as the web passes through the first few nips of a super calender. By employing a zone control roll at the top of the stack, caliper can be accurately set. Hot spots, however, occurring due to irregularities in the remaining nips of the stack, continue to occur and affect polish and bulk.
In an attempt to improve this situation, it was proposed to employ zone control rolls at the top and bottom of the super calender stack and to control the geometric profiles thereof, in tandem, as a function of caliper. See Reference No. 2 in the accompanying Prior Art Statement.
This tandem or floating system employs a pair of zone control rolls with a self-loading system in which the deformable shells float without bearing support. In this scheme the top and bottom zone control rolls are interdependent and operated by a single set of controls as a function of measured caliper of the web emerging from the calender. It is believed that the results of this system are not satisfactory due to the interdependence of the two zone control rolls. Adjustment of one affects the adjustment of the other and both are controlled only as a function of measured caliper.
The present applicants have determined that a web of superior quality can be produced by providing bearing supported top and bottom zone control rolls which are independently controlled, and wherein the top zone control roll has its geometric profile altered as a function of measured caliper of the web while the bottom zone control roll has its geometric profile altered as a function of the temperature of the bottom compliant (filled) roll. This arrangement, not known in the art, overcomes the two major problems in super calendering: (1) caliper is accurately controlled by the top zone control roll; (2) hot spots encountered in the super calender are effectively compensated for in the final nip by detecting the hot spots and modifying the geometry of the bottom zone control roll to compensate therefor.
The present invention results from discovery of the nature of the problem unsolved by the prior art and from solving that problem by recognizing that the upper zone control roll must function to set caliper while the lower zone control roll must function--independently of the upper roll-- to control the temperature profile across the final compliant roll of the super calender.
It is accordingly an object of the present invention to provide a method and apparatus for improved super calendering of web material.
Another object of the invention is to provide a method and apparatus for super calendering having a pair of independently functioning zone control rolls, one of which is controlled as a function of caliper, the other as a function of temperature.
A further object of the invention is to provide a method and apparatus which overcomes the deficiencies of the prior art by providing a variable geometry nip at the top of a super calender and a variable geometry nip at the bottom of a super calender and controlling said nips as functions of caliper and temperature, respectively.
Another object of the invention is to provide an improved super calendering device and method to obtain improved caliper bulk and polish characteristics of a web passing therethrough.
Other object and advantages of the invention will be apparent from the remaining portion of the specification.